How can I see a preview of the sequence of PNGs in the VSE?
Could be that you are looking at the wrong frame (frame zero instead of frame 1). Try advancing a frame to check.

Also check that the png is being interpreted by Blender correctly. Import just 1 frame and check.

Finally you could try using Alpha Over or Over drop in the strip's blend type. It is at the top of the strip properties bar on the right of the VSE.